Percents are easy. If you learn them the right way. If you understand Fractions and Decimal Fractions, then you know percents (it’s really percentages). After all, percentages are only fractions with "the whole" being divided to 100 equal parts.
If they are not different than fractions, and really easy, then why devote a whole 38 pages of a book to them?
Because by calling them a special name (percentages) and specifying the whole to have 100 parts, we create specific rules. Those rules are not really different than rules for fractions, but you want to remember how they work when the whole has 100 parts.
The reason many kids have problems with percents is exactly because they are not being taught as fractions but as though they are unique. Add to it that fractions are hardly ever taught correctly and fully, especially with regard to the concept of a whole, and you get a recipe for trouble.
If you want to learn Percentages the Fractions book first, then the Decimal Fractions book, and then the Percents book. You'll understand and know how to do everything with Percentages. My experience is that practically always, the problem lies not with “percents” but with lack of understanding of earlier concepts, mainly fractions and decimal fractions. Therefore, even if you think your problem is “only with percents” – study those books first, and study them thoroughly. Don’t skip because “I know that part already, I studied it in school”.
It should take you less than 5 HOURS to finish the book!